While Jennifer Aniston may be busy promoting her forthcoming movie Cake (and even garnering Oscar buzz!), the 45-year-old still finds time for the most important man in her life: fiancé Justin Theroux.

The engaged lovebirds enjoyed a date night on Monday at the Montblanc Presents 24 Hour Plays event in NYC, where the two tried to remain incognito, but couldn't hide their love for each other.

According to an eyewitness, Jen and Justin "were very discreet when they walked in and they sat in the balcony of the theater," where they watched Justin Long and Amanda Seyfried perform in one of the six plays. Following the couple's performance, Seyfried and Long joined Jen and Justin in the balcony, where both celeb couples packed on the PDA.

"Jennifer was wearing glasses and Justin was in his staple leather jacket," the insider says. "They were both sitting closely together. Justin had his hand around her and they were both laughing over the funny performances. It was a true date night!"

The eyewitness adds that "Justin and Amanda were so sweet" and showing off "major PDA," and, in a testament to Seyfried's newfound friendship withTaylor Swift, the actress even sang "Shake It Off" during her performance with her man!

Prior to their NYC date night, Jen and Justin were most recently spotted in London, where they partied together after the premiere of Aniston's upcoming film Horrible Bosses 2.

Aniston also recently opened up about her hunky man in an interview with Harper's Bazaar, admitting she couldn't be happier living life with Theroux.

"It's almost impossible to get bored with one another," the stunner said of her Wanderlust co-star. "We've tried so hard! And even that's interesting because his eyes are so pretty, but we can entertain ourselves and talk about endless things, which is pretty great."

Calling Theroux her "creative crush," Aniston gushed, "Not only is he a great actor but he's one of the best comedy writers out there. And he directs and paints murals. And I just think it's so attractive to be that good at so many things and to have no ego. He's one of the most humble, decent human beings."